Now that the function of the host subunits of Q beta replicase have been identified uninfected cells and the stages of Q beta replication are known, we will determine in which stages of replication the various subunits participate and what functions they perform in the replicase enzyme. We will select for bacterial strains carrying conditional lethal mutations in protein synthesis elongation factors Tu and Ts. Replicase made from these strains will help to elucidate the functions of these two subunits. The presence of a guanosine tetraphosphate binding site on one of the subunits of replicase would be expected to bring phage RNA replication under stringent control; yet there remains controversy concerning this question. An hypothesis is presented to explain the basis of this uncertainty and experiments are described which should resolve it. Another controversy exists about the involvement of the two light host subunits of replicase in the synthesis of stable RNA (the psi hypothesis). The T-factor mutants should establish the truth or falsehood of this hotly-disputed question. We have accumulated some evidence that an inactive form of protein synthesis elongation factor Tu exists in stationary phase cells. This finding will be further investigated. It should be possible to use our simple binding assay for Tu in order to discover and purify other proteins which interact with ppGpp and thus gain a better understanding of the proteins involved in the stringent response.